Plasma enhanced compressor duct

ABSTRACT

A compression system is disclosed, comprising a first compressor having a first flowpath, a second compressor having a second flowpath located axially aft from the first compressor, and a transition duct capable of flowing an airfow from the first compressor to the second compressor, the transition duct having at least one plasma actuator mounted in the transition duct.

BACKGROUND OF THE INVENTION

This invention relates generally to compressors, and more specificallyto a compression system having a transition duct having plasmaactuators.

In a gas turbine engine, air is pressurized in a compression moduleduring operation. The air channeled through the compression module ismixed with fuel in a combustor and ignited, generating hot combustiongases which flow through turbine stages that extract energy therefromfor powering the fan and compressor rotors and generate engine thrust topropel an aircraft in flight or to power a load, such as an electricalgenerator.

The compressor includes a rotor assembly and a stator assembly. Therotor assembly includes a plurality of rotor blades extending radiallyoutward from a disk. More specifically, each rotor blade extendsradially between a platform adjacent the disk, to a tip. A gas flowpaththrough the rotor assembly is bound radially inward by the rotor bladeplatforms, and radially outward by a plurality of shrouds.

The stator assembly includes a plurality of circumferentially spacedapart stator vanes or airfoils that direct the compressed gas enteringthe compressor to the rotor blades. The stator vanes extend radiallybetween an inner band and an outer band. A gas flowpath through thestator assembly is bound radially inward by the inner bands, andradially outward by outer bands. The rotor stages comprise rotor bladesarranged circumferentially around a rotor hub. Each compression stagecomprises a vane stage and a rotor stage.

Modern high by-pass ratio gas turbine engines have a booster (lowpressure compressor) and a high pressure compressor with a transitionduct located in between. Conventional transition or gooseneck ductgeometries are governed by their levels of endwall curvature, sinceexcessive curvature leads to endwall boundary layer separation andtherefore high losses in efficiency. To ensure a smooth aerodynamictransition without flow separation, conventional transition duct designsmust have some minimum axial length for a given change in annular flowradius. This is not desirable because increased transition duct lengthstranslate directly to increased engine length, which in turn adds engineweight and reduces backbone stiffness of the engine. This reduction instiffness makes it more difficult to maintain the desired clearancesover the rotor tips, reducing the efficiency and operability range ofthe engine.

As compressor and booster rotors approach the limits of their capabilityto add work/pressure to the air, they tend to become less efficient and,if pushed beyond this limit, stall (fail to produce their requiredpressure rise, leading to reversed flow through the stage and a loss ofengine thrust). A booster rotor that is designed very near to its limitsin the rear stages of the booster could experience significantoperability problems. This is a concern in conventional booster systemdesigns which are limited to lower radii in the aft rotor stages. Thesecould be corrected by pushing the back end of the booster outwards, asenabled by the use of plasma actuators in the transition duct.

Accordingly, it is would be desirable to have a shorter transition ductdesign having enhanced pressure distribution without causing flowseparation in the duct. It would be desirable to have a booster systemwhich has a higher radius for aft rotor stages without causing flowseparation in the transition duct.

BRIEF DESCRIPTION OF THE INVENTION

The above-mentioned needs may be met by exemplary embodiments whichprovide a compression system comprising a first compressor having afirst flowpath, a second compressor having a second flowpath locatedaxially aft from the first compressor, and a transition duct capable offlowing an airfow from the first compressor to the second compressor,the transition duct having at least one plasma actuator mounted in thetransition duct.

In another aspect of the present invention, a duct comprises an inletportion, an exit portion located at a distance axially aft from theinlet portion, an axially arcuate inner wall extending between the inletportion and the exit portion, an axially arcuate outer wall extendingbetween the inlet portion and the exit portion, an axially arcuateflowpath between the inner wall and the outer wall, and at least oneplasma actuator mounted in the duct.

In another aspect of the present invention, a gas turbine enginecomprises a duct located between a first compressor and a secondcompressor, the duct having at least one plasma actuator mounted in theduct.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding part of thespecification. The invention, however, may be best understood byreference to the following description taken in conjunction with theaccompanying drawing figures in which:

FIG. 1 is a cross-sectional view of an exemplary gas turbine engineassembly comprising a compression system according to an exemplaryembodiment of the present invention.

FIG. 2 is an enlarged axial cross-sectional view from FIG. 1 showing aportion of a booster system according to an exemplary embodiment of thepresent invention.

FIG. 3 is a schematic view of a gooseneck duct having plasma actuatorsaccording an exemplary embodiment of the present invention.

FIG. 4 is an enlarged axial cross sectional view of a portion of anexemplary duct having a plasma actuator system in the energized mode.

FIG. 5 is a schematic view of a gooseneck duct of a booster systemaccording to an exemplary embodiment of the present invention.

FIG. 6 is a schematic view of a booster system having plasma actuatorsaccording to an exemplary embodiment of the present inventionsuperimposed with a conventional booster flow path for comparison.

FIG. 7 is a plot of pressure distributions in a booster system accordingto an exemplary embodiment of the present invention when plasmaactuators are energized and de-energized.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 shows across-sectional view of an exemplary gas turbine engine assembly 10having a longitudinal axis 11 and a compression system 20 comprising afirst compressor 21 and a second compressor 22 that is located axiallyaft from the first compressor 21. In the exemplary embodiment shown inFIG. 1, the first compressor 21 is a booster 40, that is also referredto alternatively herein as a low-pressure compressor. The exemplarybooster 40 shown in FIGS. 1 and 2 has four rotor stages, with each rotorstage having between 50 and 90 booster rotor blades. The exemplarybooster system 50 has a row of stator vanes (alternatively referred toherein as booster inlet guide vanes “IGV”) located axially forward fromthe first booster rotor stage. The exemplary booster system 50 has a rowof stator vanes (alternatively referred to herein as booster outletguide vanes 44 “OGV”) located axially aft from the last booster rotorstage. The OGV 44 has 120 vanes circumferentially spaced around thelongitudinal axis 11. Further, the second compressor 22 shown in FIG. 1is an axial-flow high-pressure compressor 14 (“HPC”). The exemplary HPC14 shown in FIGS. 1 and 2 has seven rotor stages, with each rotor stagehaving between 24 and 96 HPC rotor blades. The exemplary HPC 14 has acircumferential row of 40 stator vanes (alternatively referred to hereinas HPC inlet guide vanes “IGV”) located axially forward from the firstHPC rotor stage. The exemplary embodiment of the gas turbine engineassembly 10 shown in FIG. 1 further comprises a combustor 16, and ahigh-pressure turbine 18 and a low-pressure turbine 19 that is coupledaxially downstream from core gas turbine engine 12, and a fan assembly13 that is coupled axially upstream from core gas turbine engine 12. Fanassembly 13 includes an array of fan blades 17 that extend radiallyoutward from a rotor disk 29. In the exemplary embodiment shown in FIG.1, the fan assembly 13, the booster 40 and low-pressure turbine 19 arecoupled together by a first rotor shaft 28, and compressor 14 andhigh-pressure turbine 18 are coupled together by a second rotor shaft27.

In operation, air flows through fan assembly blades 17 and a portion ofthat air flows as bypass airflow 15 and a portion of the air flows ascore airflow 25 into the compression system 20 that includes a firstcompressor 21 and a second compressor 22. In the exemplary embodimentsshown in FIGS. 1 and 2, the first compressor 21 is a booster 40 (lowpressure compressor) and the second compressor 22 is a high-pressurecompressor 14. The core airflow 25 entering the compression system 20 isfirst channeled through a first flow path 23 and is compressed in thefirst compressor 21 (shown in the figures herein as booster 40). Thecore airflow 25 is then channeled through an arcuate third flowpath 33in a duct 30 (alternatively referred to herein as a transition duct 30or as a gooseneck duct 38) to a second flowpath 24 in the secondcompressor 22 (shown in the figures herein as a high-pressure compressor14) wherein the core airflow 25 is further compressed. Airflow exitingfrom the compression system 20 is channeled to a combustor 16. Air ismixed with fuel in the combustor and burned. Products of combustion fromcombustor 16 are utilized to drive a high pressure turbine (HPT) 18 anda low pressure turbine (LPT) 19. In the exemplary embodiments shownherein, the LPT 19 drives the booster 40 and fan assembly 13 via fanrotor shaft 28 and the HPT drives the high-pressure compressor 14 via HProtor shaft 27. Engine 10 is operable at a range of operating conditionsbetween design operating conditions and off-design operating conditions.In the exemplary embodiments shown in FIGS. 1 and 2, the booster 40rotor may have operating speeds between 1500 rpm and 2700 rpm, and thehigh-pressure compressor 14 rotor may have operating speeds between 6000rpm and 12000 rpm.

In the exemplary embodiments shown in FIGS. 1 and 2, the pitchlines ofthe booster rotor stages are located radially at a higher radius thanthe pitchlines of the high-pressure compressor rotor stages. This isespecially true in the case of modern high bypass ratio engines. As usedherein, “pitchline” of a rotor stage is defined as an axial line passingthrough the radial mid-point between the root and tip of the leadingedge of the airfoil of a rotor blade in the rotor stage. The transitionduct 30 flows the core airflow 25 from the first flowpath 23 of thebooster 40 to the second flowpath 24 of the high-pressure compressor.FIGS. 3 and 5 show schematically an axial cross sectional view of anexemplary embodiment of a transition duct 30 according to the presentinvention. The terms “duct”, “transition duct” and “gooseneck duct” havethe same meaning, and are used interchangeably herein. The duct 30comprises an inlet portion 34 and an exit portion 35 that is locatedaxially aft from the inlet portion. The inlet portion 34 has an inletend 47 having an inlet area 36 and the exit portion 35 has an exit end48 having an exit area 37. The inlet portion 34 is axially located nearthe booster 40 and the exit portion is axially located near thehigh-pressure compressor 14. The inlet portion 34 is located radiallyoutward from the exit portion 35 and centerline axis 11. The duct 38comprises an inner wall 31 and an outer wall 32 that form the flowpath33 in between. The duct 38 may have an annular shape around thelongitudinal axis 11. In the exemplary embodiments shown in FIGS. 1, 2and 3, struts 46 of a support frame extend radially through the thirdflowpath 33 of the duct 38 at some circumferential locations. The thirdflow path has a generally annular shape with respect to the longitudinalaxis 11 in the axial direction, with the struts 46 extending through itat certain circumferential locations in some applications. Due to thegenerally annular configuration of the duct 38 with the inlet portion 34located radially outward from the exit portion 35, the third flow path33 and the duct 38 have a gooseneck shape, such as shown, for example,in FIGS. 3 and 5. The inner wall 31 and the outer wall 32 have anarcuate shape in the axial direction, such as shown, for example, inFIGS. 3 and 5.

Referring to FIG. 5, the inlet end 47 of the duct 30 is located at ahigher radius with respect to the longitudinal axis 11 than the exit end48. The exit end 48 is located at an axial aft distance 76 (“D”) fromthe inlet end 47. In the exemplary embodiments of the present inventionshown herein, the ratio of the inlet outer radius 71 (“RI”) to the exitouter radius 72 (“RO”) is about 1.8. For the same duct axial length,this ratio is about 1.6 or less for conventional designs. In theexemplary embodiments shown herein, the axial distance D 76 is betweenabout 16 inches and 18 inches. In the exemplary embodiments shownherein, the inlet area 36 is about 598 sq. inches and the exit area 37is about 570 sq. inches. A slight reduction in the exit area 37 from theinlet area 36 may help to further reduce flow separation in the duct 38.In other embodiments of the present invention, the inlet area 36 and theexit area 37 may have other suitable values. In alternative embodiments,it may be advantageous to have the exit area 37 larger than the inletarea 36 to improve pressure distributions in the duct 38 using knowndesign methods. The present invention enables the design of boostersystems having short duct axial lengths (“D”) as compared to the inletand exit radii (“RI” and “RO”). In the exemplary embodiments of thepresent invention, the aspect ratio, defined as the ratio (RI-RO)/D, isbetween about 0.5 and 0.8. Due to the geometric nature of the crosssectional shape of the third flowpath 33, such as shown in FIGS. 3 and5, the transition duct 30 is alternatively referred to herein as agooseneck duct 38.

As is evident from the exemplary embodiments shown herein, the innerwall 31 and outer wall 32 have significant curvatures in the axialdirection. In the exemplary embodiments of the present invention shownin FIGS. 1, 2, 3, 5 and 6 flow separation in the duct 38 is reduced byusing plasma actuators 60. The terms “plasma actuator” and “plasmagenerator” as used herein have the same meaning and are usedinterchangeably. The plasma actuators, such as for example, shown asitems 60, 61, 62, and 63 in the figures herein, strengthen the localaxial momentum of the airflow near the walls 31, 32 and minimize flowseparation in the duct 30 in regions having sharp radius of curvature inthe inner and outer walls 31, 32. Plasma actuators used as shown in theexemplary embodiments of the present invention, produce a stream of ionsand a body force that act upon the fluid near the walls 31, 32, forcingit to flow closer to the walls 31, 32 in direction of the desired fluidflow with reduced flow separation from the walls 31, 32.

FIG. 4 schematically illustrates, in axial cross-section view, anexemplary embodiment of plasma actuator 60 for reducing the flowseparation in a transition duct 38 located between two compressors, suchas the booster 40 and the HPC 14 shown in FIGS. 1 and 2. The exemplaryembodiments of the present invention shown herein facilitate animprovement of the pressure distribution in the duct 38 (see FIG. 7)and/or enhance the efficiency of compression systems, in a gas turbineengine 10 such as the aircraft gas turbine engine illustrated incross-section in FIG. 1. The exemplary gas turbine engine plasmaactuators shown in FIGS. 1-6 include plasma actuators, such as shown asitems 60, 61, 62 or 63 located on the inner wall 31, outer wall 32 orthe hub portion 45 of the booster OGV 44. The plasma actuator, such asitem 60 shown in FIG. 4, is located in a groove 68 in a wall, such asthe inner wall 31. The plasma actuator 60 may be continuous in thecircumferential direction located in an annular groove. Alternatively,the plasma actuator 60 may be segmented wherein a plurality of plasmaactuators 60 are located in corresponding groove segments spacedcircumferentially in the walls 31, 32. The exemplary embodiment shown inFIG. 4 comprises a plasma actuator 60 located in a groove 68 in theinner wall 31 of the duct 38. Alternately, the plasma actuators 60 maybe located at other locations in the duct 38 where flow separation islikely to occur, such as, for example, locations where the duct 38 wallshave a sharp radius of curvature in the direction of airflow.

The exemplary embodiment shown in FIG. 4 shows an annular plasmagenerator 60 mounted to the inner wall 31 and includes a first electrode64 and a second electrode 66 separated by a dielectric material 65. Thedielectric material 65 is disposed within an annular groove 68 of theduct 38. An AC (alternating current) power supply 70 is connected to theelectrodes 64, 66 to supply a high voltage AC potential in a range ofabout 3-20 kV to the electrodes 64, 66. When the AC amplitude is largeenough, the air ionizes in a region of largest electric potentialforming a plasma 80. The plasma 80 generally begins near an edge 67 ofthe first electrode 64 which is exposed to the air and spreads out overan area 69 projected by the second electrode 66 which is covered by thedielectric material 65. The plasma 80 (ionized air) in the presence ofan electric field gradient produces a force on the airflow 25 near thewall 31 inducing a virtual aerodynamic shape that causes a change in thepressure distribution over the inner wall 31 of the annular duct 38. Theair near the electrodes is weakly ionized, and usually there is littleor no heating of the air. The airflow 25 near the wall 31 tends toremain attached to the wall 31 resulting in reduced flow separation andimproved pressure distribution within the duct 38 due to reducedpressure loss in the duct 38.

FIG. 6 shows a booster system 50 according to an exemplary embodiment ofthe present invention. The booster system 50 shown in FIG. 6 has a lastrotor stage 57 having a pitchline radius 54 that is larger thanconventional booster systems. This is made possible in the presentinvention by the use of plasma actuators, such as, for example, shown asitems 60, 61, 62 in FIG. 6, in a duct 38 that receives the flow from thelast stage of the booster. A conventional flowpath 90 of a conventionalbooster system is shown by dotted line in FIG. 6 for comparison with theexemplary embodiment of the present invention, booster system 50. Thereare several benefits associated with having the aft stages of thebooster, such as the last rotor stage 57, radially further outward. Therotor stage 57, having a larger pitchline radius 54, has an increasedtip speed compared to conventional designs. Since the ability of a rotorto do work on a fluid is directly related to its tangential velocity,the exemplary embodiment of the present invention shown in FIG. 6 hasincreased capacity to produce pressure rise. In some applications, for adesired pressure ratio, it is possible to reduce the number of requiredstages in a booster system by using the present invention resulting insignificantly reduced weight for the engine 10.

In the exemplary embodiment of the present invention shown in FIG. 6,the booster system 50 has a first rotor stage 55 comprising a pluralityof first rotor blades 56 spaced circumferentially around a rotor hub 41and having a first pitch-line radius 53 extending from the longitudinalaxis 11, a last rotor stage 57 located axially aft from the first rotorstage 55. The last rotor stage 57 has a plurality of last rotor blades58 spaced circumferentially around the rotor hub 41 and has a secondpitch-line radius 54 extending from the longitudinal axis 11. Thebooster system has a gooseneck duct 38 located axially aft from the lastrotor stage 57 and receives the airflow 25 exiting from the last rotorstage 57. The gooseneck duct 38 has an inlet end 47, an exit end 48located at a distance axially aft from the inlet end 47, and has atleast one plasma actuator mounted in the gooseneck duct 38. The geometryof the gooseneck duct 38 and the placement of the plasma actuators, suchas for example, shown as items 60, 61, 62 in FIG. 6, are describedpreviously herein. Unlike conventional booster systems, the last rotorstage 57 has a higher pitchline radius 54 “B” as compared to the firstrotor stage 55 pitchline radius 53 “A”. In the exemplary embodiments ofthe present invention shown herein, the ratio B/A is at least 0.9.

The exemplary booster system 50 shown in FIG. 6 has a gooseneck duct 38located at the aft end, the duct 38 having an axially arcuate inner wall31 and an axially arcuate outer wall 32. The exit end 48 has an exitarea 37 and the inlet end 47 has an inlet area 36. The geometry of thegooseneck duct (see FIG. 5) is such that the ratio RI/RO of the inletouter radius 71 to the exit outer radius 72 is at least 1.6. Plasmaactuators, such as for example, shown as items 60, 61, 62 in FIG. 6 arelocated in the duct 38 as described previously herein.

A gas turbine engine 10 having a booster system 50 with the gooseneckduct 38 having plasma actuators as described herein, can be operated byenergizing the first electrode 64 and second electrode 66 using the ACpotential from the AC power supply 70. By energizing the electrodes 64,66 and creating the plasma 80, flow separation in the duct 38 can bereduced which results in the advantages and improvements in pressuredistributions in the booster system 50. In one method, the plasmaactuators, such as item 60 in FIG. 6, can be energized continuouslythroughout engine operation period. Alternatively, the plasma actuatorscan be energized only during selected portions of the engine operatingregime. The periods and durations of plasma actuator energization can bedetermined by known engine test methods for determining engineoperability.

FIG. 7 shows an exemplary pressure distribution within the duct 38 atthe exit end 48 determined by known fluid flow analytical methods. Thehorizontal axis shows the normalized pressure and the vertical axisshows the radial span locations within the duct 38. The distributionidentified by numeral 91 shows the radial pressure distribution at theexit end 48 of the duct 38 when the plasma actuator 60 is not energizedby the AC power supply 70. The distribution identified by numeral 92shows the radial pressure distribution at the same location (exit end 48of the duct 38) when the plasma actuator 60 is energized by the AC powersupply 70. It is clear that near the wall 32 (near the 1.0 spanlocation) wherein the plasma actuator is located, the normalizedpressure increases from about 0.79 to about 0.86.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralsaid elements or steps, unless such exclusion is explicitly recited.When introducing elements/components/steps etc. of designing and/ormanufacturing components and systems described and/or illustratedherein, the articles “a”, “an”, “the” and “said” are intended to meanthat there are one or more of the element(s)/component(s)/etc. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional element(s)/component(s)/etc. otherthan the listed element(s)/component(s)/etc. Furthermore, references to“one embodiment” of the present invention are not intended to beinterpreted as excluding the existence of additional embodiments thatalso incorporate the recited features.

Although the methods and articles such as vanes, outer bands, innerbands and vane segments described herein are described in the context ofa compressor used in a turbine engine, it is understood that the vanesand vane segments and methods of their manufacture or repair describedherein are not limited to compressors or turbine engines. The vanes andvane segments illustrated in the figures included herein are not limitedto the specific embodiments described herein, but rather, these can beutilized independently and separately from other components describedherein.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

While there have been described herein what are considered to bepreferred and exemplary embodiments of the present invention, othermodifications of the invention shall be apparent to those skilled in theart from the teachings herein, and it is, therefore, desired to besecured in the appended claims all such modifications as fall within thetrue spirit and scope of the invention.

1. A compression system comprising: a first compressor having a firstflowpath; a second compressor located axially aft from the firstcompressor, the second compressor having a second flowpath; and atransition duct located between the first compressor and the secondcompressor capable of flowing an airfow from the first compressor to thesecond compressor, the transition duct having at least one plasmaactuator mounted in the transition duct.
 2. A compression systemaccording to claim 1 wherein at least a portion of the second flowpathis located radially inward from a portion of the first flowpath.
 3. Acompression system according to claim 1 wherein the first compressor isa booster having a row of booster blades arranged in a circumferentialdirection around a longitudinal axis.
 4. A compression system accordingto claim 3 wherein the second compressor is an axial-flow compressorhaving a row of compressor blades arranged in a circumferentialdirection around the longitudinal axis.
 5. A compression systemaccording to claim 1 wherein the transition duct comprises an axiallyarcuate inner wall and an axially arcuate outer wall.
 6. A compressionsystem according to claim 5 wherein the inner wall and outer wall form athird flowpath having an inlet portion and an exit portion located at adistance axially aft from the inlet portion.
 7. A compression systemaccording to claim 6 wherein the inlet portion has an inlet area and theexit portion has an exit area that is greater than the inlet area.
 8. Acompression system according to claim 5 wherein the at least one plasmaactuator is located on the inner wall.
 9. A compression system accordingto claim 5 wherein the at least one plasma actuator is located on theouter wall.
 10. A compression system according to claim 1 furthercomprising an outlet guide vane located between the first compressor andthe transition duct wherein the outlet guide vane comprises a hubportion having a plasma actuator located on the hub portion.
 11. Acompression system according to claim 1 wherein the plasma actuator iscontinuous in a circumferential direction around a longitudinal axis.12. A compression system according to claim 1 further comprising aplurality of plasma actuators arranged in a circumferential directionaround a longitudinal axis.
 13. A compression system according to claim1 wherein the plasma actuator comprises a first electrode and a secondelectrode separated by a dielectric material.
 14. A compression systemaccording to claim 13 further comprising an AC power supply connected tothe first electrode and the second electrode to supply a high voltage ACpotential to the first electrode and the second electrode.
 15. A ductcomprising: an inlet portion; an exit portion located at a distanceaxially aft from the inlet portion; an axially arcuate inner wallextending between the inlet portion and the exit portion; an axiallyarcuate outer wall extending between the inlet portion and the exitportion; an axially arcuate flowpath between the inner wall and theouter wall; and at least one plasma actuator mounted in the duct.
 16. Aduct according to claim 15 wherein the inlet portion has an inlet areaand the exit portion has an exit area that is greater than the inletarea.
 17. A duct according to claim 15 wherein the at least one plasmaactuator is located on the inner wall.
 18. A duct according to claim 15wherein the at least one plasma actuator is located on the outer wall.19. A duct according to claim 15 wherein the plasma actuator comprises afirst electrode and a second electrode separated by a dielectricmaterial.
 20. A duct according to claim 13 further comprising an ACpower supply connected to the first electrode and the second electrodeto supply a high voltage AC potential to the first electrode and thesecond electrode.
 21. A gas turbine engine comprising a duct locatedbetween a first compressor and a second compressor, the duct having atleast one plasma actuator mounted in the duct.